Ultra-fast photoinduced gigantic metallization in quarter filled organic A 2 B salts: (EDO-TTF) 2 PF 6 Laurent Guérin, Matthieu Chollet, N. Uchida, S. Fukaya, T. Ishikawa,S. Koshihara, Tokyo Institute of Technology A. Ota, H.Yamochi, G. Saito, Kyoto University Marylise Buron, Eric Collet, Hervé Cailleau, GMCM, université de Rennes 1
Photoinduced Phase Transition Out of equilibrium processes at different scales lattice relaxation Ground state Nanometric New lattice structure and electronic order Nasu K. and al, J. phys : Condens. Matter (2001)
Metal insulator transition in (EDOTTF) 2 PF 6 (EDOTTF) 2 PF 6 A noble Metal Insulator Transition accompanied with multi-instability at 280K A. Ota, J. Mater. Chem. 12 (2002)
A novel M-I transition RT (27) (27) 260K (-23) (13) (27) (-23) Multi-instability: Metal Insulator Peierls transition Charge ordering Anion ordering
How can the transition be probed? Large change in the 0.8 1.8 ev region (important for application) Time-resolved optical spectroscopy Femtosecond laser 70 fs - pump : 800 nm - probe : continuum Access to different times of the transition with 100 fs resolution
Photoconductivité (arb. units) Jump of the photoconductivity Insulator to metal transition associated with change of the reflectivity Large change of the reflectivity easily probed using CCD camera Chollet et al. Science 307, 84 (2005)
Spectral changes CO melting accompanied by I-M phase conversion occurs within 3ps Fast time dependance in reflectivity change 6. 10 18 photons.cm -3 Phase conversion process completed in 1.5 ps Highly efficient conversion : 50% change 1 photon/500molecules Strong cooperativity
Excitation intensity dependance Nonlinear response is observed 1.38eV 3ps Threshold like behavior : 1photon/ 1500 molecules Internal cooperative interactions Life time depends on photoexcitation energy Cooperative interactions occurs in the relaxation process Quick recovery time for application in phase- switching devices
Vibrational structure of the photo-induced I-M transition Period of vibration : independant of probe photon energy Temperature dependant Softening of the phonon mode while Increasing temperature Chollet et al. Science 307, 84 (2005)
Mechanism of the photo-induced transition Hasegawa, thesis (2004) The optical phonon mode corresponds to the bending mode of EDOTTF molecules Strong coupling between CO and molecular deformation Incoherent vs coherent phonon process : Speed of domain growth limited by incoherent process Meso-size domain mediated by coherent phonon induced just after photo-excitation
Coherent optical phonons observed in Bismuth K. Sokolowski-Tinten et al., Nature 422, 287 (2003) F(hkl) 2 (222) (111) initial equilibrium position d=0.5 displaced quasi-equilibrium position d, distance between atoms 1 and 2 Weak rhombohedral distortion of the CFC unit-cell Atom 1 at (0,0,0), atom 2 close to (0.5,0.5,0.5)
Summary (EDO-TTF) 2 PF 6 crystal shows highly sensitive (50% efficiency for weaker excitation intensity than 1photon/500 molecules) I (CO)-to-M like PIPT within 1 ps Such highly sensitive and ultra-fast response may be characteristics of molecular deformation (lattice) charge spin coupling in ¼ filled system. Nonlinear response to excitation intensity has been observed (importance of co-operativity) E-L interaction via an optical coherent phonon mode plays an important role in the Photoinduced transition. Femto-sec. X-ray crystallography is a key for solving mechanism.
Collaborators : Department of Materials Science Tokyo Institute of Technology: Matthieu Chollet T. Hasegawa Shin-ya Koshihara Funds : Collège Doctoral Franco-Japonais GMCM Université de Rennes 1 : Marylise Buron Eric Collet Hervé Cailleau Kyoto University : A. Ota G. Yamochi G. Saito ID09B ESRF : Michael Wulff Maciej Lorenc KEK Tsukuba : Shin-ichi Adachi Shunsuke Nozawa Jun-ichi Takahashi Ryoko Tazaki
Cycle between I state and M photoinduced state 0.56eV 1.36eV Excitation by 1.56eV Induced instability Bent molecules Flat molecules Bent molecules Flat molecules Free charge trapping Lattice relaxation Vibration Bent molecules Bent molecules Bent molecules Flat molecules Hasegawa, thesis (2004)